Application of electrical currents to modify brain function has been practiced for a very long time. Systematic animal studies in anesthetized rats demonstrated that weak direct currents, delivered by intracerebral or epidural electrodes, induce cortical activity and excitability diminutions or enhancements, which can be stable long after the end of stimulation. The long-lasting effects can be used to alter neural activity and behavior. Initial studies in humans aimed at treating or modifying psychiatric diseases, particularly depression, suggested diminished depressive symptoms, and reduced manic symptoms. In the last few decades, TDCS was re-evaluated and shown to reliably modulate human cerebral cortical function inducing focal, prolonged but yet reversible shifts of cortical excitability.
Studies combining TDCS with other brain imaging and neurophysiologic mapping methods (such as MRI, PET, EEG) promise to provide invaluable insights on the correlation between modification of behavior and its underlying neurophysiologic underpinnings. Depending on where the anode and cathode electrodes are placed on the head of a patient, studies show that various disorders and behaviors can be treated using TDCS by stimulating different parts of the brain. In some instances, the polarity of the electrodes, along with the placement, can affect the type of condition and area of the brain to be treated.
Current electrodes used with TDCS often use a carbon or steel mesh electrode with a sponge and saline or other medium to achieve an electrical connection between the electrodes through skin (and/or hair). However, the electrodes used tend to cause skin irritation because the current tends to achieve some degree of hydrolysis in the saline resulting in a pH change in the saline solution (or other conductive medium), which tends to irritate the skin causing discomfort to the patient.